1. Field of the Invention
The present invention relates to an improved process for producing a metal-screened ceramic package including screening a metal on the green ceramic sheet and co-firing the resulting metal-screened sheet.
2. Description of the Prior Art
Typically, the simultaneous sintering of ceramic is performed in the presence of the mixture gas of hydrogen and nitrogen in a state of humidity wherein the mixture ratio of hydrogen and nitrogen and the dew point of humid mixture gas can control the oxygen partial pressure in the furnace. The controlled oxygen partial pressure according to this method is a function of many parameters including the combustion characteristics of binders during the co-firing process and the sintering characteristics of the metal-ceramic.
FIG. 1 illustrates in typical perspective view a ceramic package, showing a typical process for printing a metal layer on a typical green ceramic sheet. In general, a green ceramic sheet 1 is made by mixing alumina powder with SiO.sub.2, CaO, and MgO, which are the main constituents of glass, blending them with a synthetic resin binder such as a hydrocarbon, plasticizer, and solvent, moulding the resulting mixture as a sheet, and drying the resulting green ceramic sheet. A metal pattern is formed on the resulting typical green ceramic sheet by screening onto a surface of the sheet a metal paste which is made by mixing a high melting point metal powder such as tungsten, molybdenum of the like, with binder and solvent. And the resulting metal-screen green ceramic sheet is co-fired at 500.degree.-1650.degree. C. in the presence of hydrogen and nitrogen mixture gas including a given amount of humidity.
At the same time a, glass phase, which is feasible to be wet and is formed on the ceramic by oxidization on the surface of the metal, is moved into the metal layer by the capillary phenomena which is formed between metal particles. The transferred glass phase accelerates the sintering of metal particles depending on the sintering mechanism, such as rearrangement of metal and reextraction of the solution.
Furthermore, the transferred glass phase is solidified during cooling of the fired, metal-screened ceramic sheet, so the adhesive strength of the interface is improved.
During co-firing of metal and ceramic, it is most desirable to have the shrinkage of the metal parts and ceramic parts to be matched. If the shrinkage rates of the metal and ceramic parts are different, the difference causes delamination between the layers, warpage, and weakness of the adhesive strength of the interface due to the stress.
In case of typical paste, it has been tried to change the conditions of metal particle size and distribution, solids in the paste, addition of additives, and the multilayer and sintering environment. U.S. Pat. No. 4,109,377 (Blazick et. al) provided a process described below to form multilayer ceramic substrates. It deposits a particulate mixture of a metal and the oxide thereof in the ratio of between about 1:1 to 9:1 in a pattern on at least a portion of unfired ceramic layers in order to prevent cracking of the ceramic by keeping the shrinkage rate consistent when the metal-coated ceramic is fired. In Blazick et al., the metal has the same thermal properties, such as high conductivity and heat expansion coefficient as the ceramic has. The patterns are then dried. The plurality of substrates or layers of ceramic are then laminated under a substantial pressure (2500 lb/in.sup.2) and fired at an elevated temperature in a non-oxidizing atmosphere and, finally laminated ceramic is then cooled at room temperature.
However, it has been very disadvantageous to have to control the, conditions in the furnace very sophisticatedly because the shrinkage rate of the paste produced by the typical process described above is so delicate, depending upon the oxygen partial pressure. Transport phenomena resulting from wetting and capillary properties in the glass phase are changed and, as a result, deviations in the sheet resistance, shrinkage rate, and the strength of the interface are tremendous.